Hydrothermal vents

Hydrothermal vent communities were first discovered in 1977, at a depth of 2 500 m on the Galapagos Rift but occur in many areas of tectonic activity. Cold bottom water permeates through fissures in the ocean floor close to ocean floor spreading centres, becomes heated at great depths in the Earth's crust and finds its way back to the surface through hydrothermal vents. The temperature of vent water varies greatly, from around 23 �C in the Galapagos vents to around 350 �C in the vents of the East Pacific Rise, and they may be rich in metalliferous brines and sulphide ions. Most species live out of the main flow at temperatures of around 2 �C, the ambient temperature of deep sea water. The biomass of vent communities is usually high compared with other areas of similar depth, and dense colonies of tube worms, clams, mussels and limpets typically constitute the major components.

Hydrothermal vent communities are of particular interest because they flourish in the dark at high pressures and low temperatures, and unique because they are supported by chemolithoautotrophic archeans and bacteria, notably Thiomicrospira species, which form dense microbial carpets and derive their energy chiefly from oxidising hydrogen sulphide. Many of the eukaryote vent species filter-feed on these microorganisms, whilst others rely on symbiotic sulphur bacteria for energy.

The overall species diversity at vents is low compared with other deep-sea soft sediment areas, but endemism is high. More than 20 new families or sub-families, 50 new genera and nearly 160 new species have been recorded from vent environments, including brine and cold seep communities. Vent communities are separated by gaps of between 1 and 100 km, and although they may persist only for several years or decades, sites of vent activity move relatively slowly allowing dispersal of vent organisms.